Expression of genes encoding neuropeptides, receptors and enzymes in the brain, with emphasis on the hypothalamus, are being studied. We successfully created a knock-out of the vasopressin 1b receptor (V1bR) gene in mice through homologous recombination. These mice are growing normally and we are studying them using a variety of different physiological and behavioral challenges. They are not as aggressive as their wildtype littermates, which presents the opportunity to examine the neurological basis of aggression. They also show mild reduction in social recognition. Various anatomical and physiological parameters are under study currently. We are also studying the vasopressin 1a receptor (V1aR) knockout that, surprising, shows no reduction in aggression. Other aspects of this mouse's phenotype are in early steps of exploration. Double knockouts of the V1aR and V1bR are viable and outward appearances are normal. They will be studied behaviorally and physiologically, as will an oxytocin receptor knockout that we have recently developed. We are also examining the areas of expression of the V1bR gene in the rodent brain by immunohistochemistry, hybridization histochemistry and 2-deoxyglucose uptake.